Inheritance

It is important to remember that all body cells (in situations that you are likely to come across) will be diploid. In humans (except in red blood cells) there are 46 chromosomes in all body cells - 23 pairs. Each pair of chromosomes is numbered and has its own particular genes.

In gametogenesis, (the production of sperm and eggs) this number is reduced to 23.

Only one chromosome of a pair can be inherited. Gametes are haploid. Which chromosome of the pair is inherited is random (see Independent Assortment in Meiosis). When working out the chances of an offspring inheriting a particular genotype, this fact must be remembered.

Drosophila (fruit flies) can be either straight-winged or curved-winged, this characteristic is controlled by one pair of genes. When straight- and curved-winged are bred together, all the offspring are straight-winged.

This means that straight-winged is dominant and curved-winged is recessive.

Therefore:

'straight' allele = S

'curved' allele = s

Since the allele for curved wings is recessive, if a fly has curved wings, it must have 2 alleles for curved wings = ss

Since the allele for straight wings is dominant, a straight winged fly will have either SS alleles or Ss alleles.

Question 1:

What would be the result in the F1 generation of crossing a homozygous straight-winged fly with a curved-wing fly?

The easiest way to show what the offspring will look like, is to work through this sequence:

All F1 offspring are Ss.

This means that for each offspring there is a 100% probability that they will be Ss and therefore straight-winged.

Question 2:

What would be the result in the F2 generation of crossing 2 of the F1 flies?

This means that for each offspring there is a 75% probability that they will be straight-winged and 25% probability that they will be curved-winged.

Another way of saying this is ratio of straight : curved is 3 : 1.

It does not necessarily mean that out of 4 offspring, 3 would definitely be straight-winged and 1 would be curved-winged. It is possible, though unlikely, that all offspring could be curved or straight-winged.

Question 3:

How would you determine the genotype of any unknown straight-winged fly?

To find out whether a genotype is homozygous dominant (SS) or heterozygous (Ss), a test-cross needs to be done since you cannot tell the genotype by looking at the fly.

The unknown is bred with a known. The only phenotype that gives a known genotype is homozygous recessive (ss).